Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D7: Acoustics II: General |
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Chair: Pablo Rendon, Universidad Nacional Autonoma de Mexico Room: 107 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D7.00001: Schlieren imaging of the standing wave field in an ultrasonic acoustic levitator Pablo Luis Rendon, Ricardo R. Boullosa, Carlos Echeverria, David Porta We consider a model of a single axis acoustic levitator consisting of two cylinders immersed in air and directed along the same axis. The first cylinder has a flat termination and functions as a sound emitter, and the second cylinder, which is simply a refector, has the side facing the first cylinder cut out by a spherical surface. By making the first cylinder vibrate at ultrasonic frequencies a standing wave is produced in the air between the cylinders which makes it possible, by means of the acoustic radiation pressure, to levitate one or several small objects of different shapes, such as spheres or disks. We use schlieren imaging to observe the acoustic field resulting from the levitation of one or several objects, and compare these results to previous numerical approximations of the field obtained using a finite element method. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D7.00002: Large eddy simulation of trailing edge noise Jacob Keller, Zane Nitzkorski, Krishnan Mahesh Noise generation is an important engineering constraint to many marine vehicles. A significant portion of the noise comes from propellers and rotors, specifically due to flow interactions at the trailing edge. Large eddy simulation is used to investigate the noise produced by a turbulent 45 degree beveled trailing edge and a NACA $0012$ airfoil. A porous surface Ffowcs-Williams and Hawkings acoustic analogy is combined with a dynamic endcapping method to compute the sound. This methodology allows for the impact of incident flow noise versus the total noise to be assessed. LES results for the 45 degree beveled trailing edge are compared to experiment at $M=0.1$ and $Re_c=1.9e6$. The effect of boundary layer thickness on sound production is investigated by computing using both the experimental boundary layer thickness and a thinner boundary layer. Direct numerical simulation results of the NACA $0012$ are compared to available data at $M=0.4$ and $Re_c=5.0e4$ for both the hydrodynamic field and the acoustic field. Sound intensities and directivities are investigated and compared. Finally, some of the physical mechanisms of far-field noise generation, common to the two configurations, are discussed. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D7.00003: Sound produced by subcritical Reynolds number cylinder flow Zane Nitzkorski, Krishnan Mahesh Sound production from cylinders has been studied due to their canonical application to investigating bluff body flow noise. The effect of Reynolds number for circular cylinders for Re$=$3900, 10000, and 89000 are investigated with the resulting impact on the noise generation process. The physics of noise production are investigated and a model for understanding the source and direction of noise propagation is presented. The acoustic solution is calculated from a novel porous Ffowcs-Williams and Hawkings acoustic analogy which is described and allows for investigating the scattered and incident acoustic fields by separating volume contributions from the total noise. The volume source terms are correlated over multiple planes to obtain a convection velocity that is then used to determine a corrective convective flux at the FW-H porous surface. The hydrodynamic fields are validated and the computed sound is compared with experiments. The effect of spanwise coherence and its effect on the physics of sound production is discussed. For the highest Reynolds number case, a dynamic mode decomposition is performed on the acoustic sources to demonstrate their spatial distribution and net effect. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D7.00004: Eulerian Simulation of Acoustic Waves Over Long Range in Realistic Environments Subhashini Chitta, John Steinhoff In this paper, we describe a new method for computation of long-range acoustics. The approach is a hybrid of near and far-field methods, and is unique in its Eulerian treatment of the far-field propagation. The near-field generated by any existing method to project an acoustic solution onto a spherical surface that surrounds a source. The acoustic field on this source surface is then extended to an arbitrarily large distance in an inhomogeneous far-field. This would normally require an Eulerian solution of the wave equation. However, conventional Eulerian methods have prohibitive grid requirements. This problem is overcome by using a new method, ``Wave Confinement'' (WC) that propagates wave-identifying phase fronts as nonlinear solitary waves that live on grid indefinitely. This involves modification of wave equation by the addition of a nonlinear term without changing the basic conservation properties of the equation. These solitary waves can then be used to ``carry'' the essential integrals of the acoustic wave. For example, arrival time, centroid position and other properties that are invariant as the wave passes a grid point. Because of this property the grid can be made as coarse as necessary, consistent with overall accuracy to resolve atmospheric/ground variations. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D7.00005: Roles of a scatter on boundary-layer instability and acoustic radiation Ming Dong, Xuesong Wu When a boundary-layer instability mode propagates through a region of rapid distortion, the ensuing scattering causes two consequences of physical interest. First, the amplitude of the instability mode may be suppressed or energized. Second, substantial sound wave can be radiated by the boundary-layer instability mode. This paper focuses on this issue by proposing a framework which is called Local Scattering Theory. In this framework, a transmission coefficient, defined as the ratio of the T-S wave amplitude downstream of the scatter to that upstream, is introduced to characterize the effect of a local scatter on boundary-layer instability and transition. The mathematical formulation is based on triple-deck formulism, but in order to accommodate the acoustic far field, the unsteady terms in the upper deck are retained. By computation, the impacts of a steady local suction on flow instability and acoustic radiation are studied. It is found that, (1) a suction slot would suppress the oncoming T-S wave; (2) the acoustic waves radiated by the scattering effect have similar directivities; (3) the intensity of the sound increases with the mass flux when the latter is not too large, and it also increases with the frequency monotonously. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D7.00006: Acoustic Localization with Infrasonic Signals Arnesha Threatt, Brian Elbing Numerous geophysical and anthropogenic events emit infrasonic frequencies (\textless 20 Hz), including volcanoes, hurricanes, wind turbines and tornadoes. These sounds, which cannot be heard by the human ear, can be detected from large distances (in excess of 100 miles) due to low frequency acoustic signals having a very low decay rate in the atmosphere. Thus infrasound could be used for long-range, passive monitoring and detection of these events. An array of microphones separated by known distances can be used to locate a given source, which is known as acoustic localization. However, acoustic localization with infrasound is particularly challenging due to contamination from other signals, sensitivity to wind noise and producing a trusted source for system development. The objective of the current work is to create an infrasonic source using a propane torch wand or a subwoofer and locate the source using multiple infrasonic microphones. This presentation will present preliminary results from various microphone configurations used to locate the source. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D7.00007: Nozzleless Spray Cooling Using Surface Acoustic Waves Kar Man Ang, Leslie Yeo, James Friend, Yew Mun Hung, Ming Kwang Tan Due to its reliability and portability, surface acoustic wave (SAW) atomization is an attractive approach for the generation of monodispersed microdroplets in microfluidics devices. Here, we present a nozzleless spray cooling technique via SAW atomization with key advantage of downward scalability by simply increasing the excitation frequency. With generation of micron size droplets through surface destabilization using SAW, the clogging issues commonly encountered by spraying nozzle can be neutralized. Using deionised water, cooling is improved when the atomization rate is increased and the position of the device is optimized such that the atomized droplets can be easily seeded into the upstream of the flow circulation. Cooling is further improved with the use of nanofluids; a suspension of nanoparticles in water. By increasing nanoparticle mass concentration from 1{\%} to 3{\%}, cooling is enhanced due to the deposition and formation of nanoparticle clusters on heated surface and eventually increase the surface area. However, further increase the concentration to 10{\%} reduces the cooling efficiency due to drastic increase in viscosity $\mu$ that leads to lower atomization rate which scales as $\dot{m} \sim \mu^{-1/2}$. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D7.00008: Tunable Acoustic Attenuation by Dilute Suspensions of Oblate-Spheroidal Ferromagnetic Particles Under an External Magnetic Field: An Experimental Study Wuhan Yuan, Jerry Shan, Liping Liu The microstructure of suspensions of spheroidal ferromagnetic particles with subwavelength size can be controlled by an external field, making it possible to develop novel broadband acoustic materials with anisotropic and tunable acoustic properties. In this study we experimentally show that dilute suspensions of nickel microflakes exhibit a 20{\%} to 30{\%} change in attenuation coefficient at MHz frequencies upon changing the direction of an external magnetic field, at particle volume fractions of only 0.5{\%}. Further investigations are conducted to study the mechanism behind this anisotropy. The effects of particle aligning and chaining are analyzed with the aid of optical transmission measurements. By making comparison to suspensions of spherical particles, we show that the ellipsoidal shape of the nickel microflakes plays an important role in tunable acoustic properties of these suspensions. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D7.00009: Dynamic vortex interactions with flexible fibers and edges for prediction of owl noise suppression Sarah Korykora, Justin Jaworski The compliant trailing-edge fringe of owls and the soft downy material on their upper wing surfaces are thought to enable their silent flight by weakening the interaction of boundary layer turbulence with these flexible structures. Previous analysis of turbulence noise generation by wave-bearing elastic edges have shown that the far-field acoustic power scaling can be weakened by up to the square of the Mach number relative to a rigid edge. However, it is unclear whether or not the wave-bearing feature or simply the flexible nature of the edge scatterer produces this noise suppression. To assess this distinction, a dynamic vortex interaction model is developed whereby the motion of a line vortex round a rigid but elastically-restrained wall-mounted fiber or trailing edge is determined numerically. Special attention is paid to the dynamic interaction between the flexible structure and vortex, which is accomplished via a conformal mapping relationship determined in closed form. Results from this analysis seek to develop a vortex sound model to discern the effect of flexible versus wave-bearing scatterers on turbulence noise suppression and help explain the mechanisms of silent owl flight. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D7.00010: Development of a Transient Acoustic Boundary Element Method to Predict the Noise Signature of Swimming Fish Nathan Wagenhoffer, Keith Moored, Justin Jaworski Animals have evolved flexible wings and fins to efficiently and quietly propel themselves through the air and water. The design of quiet and efficient bio-inspired propulsive concepts requires a rapid, unified computational framework that integrates three essential features: the fluid mechanics, the elastic structural response, and the noise generation. This study focuses on the development, validation, and demonstration of a transient, two-dimensional acoustic boundary element solver accelerated by a fast multipole algorithm. The resulting acoustic solver is used to characterize the acoustic signature produced by a vortex street advecting over a NACA 0012 airfoil, which is representative of vortex-body interactions that occur in schools of swimming fish. Both 2S and 2P canonical vortex streets generated by fish are investigated over the range of Strouhal number $0.2< St <0.4$, and the acoustic signature of the airfoil is quantified. This study provides the first estimate of the noise signature of a school of swimming fish. [Preview Abstract] |
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